1. Field of the Invention
The invention relates to a method and system for motor vehicle longitudinal control, and more particularly longitudinal control at low speed.
2. Description of Related Art
Driving-assistance systems, such as an automatic spacing-control system with a stop & go function, operate to control longitudinal movement of motor vehicles. Providing smooth operation and ride quality at low vehicle speeds requires controlling longitudinal vehicle movement in a gentle and jerk-free manner, since sensitivity to jerks, short abrupt motions or frequent jolts caused by rapid changes of acceleration, increases at low vehicle speed.
Controlling longitudinal vehicle movement represents a challenge, insofar as longitudinal velocity control, including set point control, is difficult to realize solely with the aid of conventional braking system since making fine or more exact torque control is necessary. In operation, a conventional braking system exhibits step-like changes in pressure causing jerks or as set forth above, short abrupt motions or frequent jolts, affecting vehicle ride and handling. It is essential that changes in brake pressure occur as gently or gradually as possible to maintain gentle and smooth longitudinal control when using the braking system. Often such smooth and gentle changes are not possible due to existing design defaults with conventional braking systems. Whereby, in practice, as a rule, jerk-free longitudinal control at low speeds using conventional braking systems is not attainable.
Using the engine for a finer or more accurate control in the conventional method for determining torque requirement, in which the engine torque is calculated and is multiplied by the transfer function of subsequent components in the drive train, is inaccurate. Finer torque control requires a more direct and robust computational method to calculate the propulsion torque. Moreover, incorrect control of the engine, and correspondingly torque, results in low-frequency oscillations during the control action in engine speed, which may be audible and troublesome for the driver as well as any other occupants of the vehicle.